The invention relates to an arrangement for receiving digital data which adapts itself to channel variations of a transmission system, comprising an adaptive receiving stage, which incorporates an adaptive equalizer and at least one sampler, the adaptive receiving stage producing, at a certain rate, complex samples y.sub.k from the input signal x(t), a decision circuit supplying detected complex symbols a.sub.k, and an adaptive timing recovery arrangement determining the optimum sampling instant by minimizing a quadratic function J which represents the difference between y.sub.k and a.sub.k.
The performance of high-speed digital transmission systems using at the receiver end an adaptive equalizer having coefficient taps spaced apart by one symbol duration to compensate for linear distortions of the channel, depend to a very large extent on the sampling instant. A bad choice of the sampling instant may result in a zero spectrum near the edge of the spectrum band of the sampled signal, in which case equalizing the channel will be difficult.
Typically, the adaptive equalizers utilize the minimum mean square error criterion for adaptation to the channel. The sampling instant is chosen at the output of the equalizer while minimizing a quadratic function J which, also and preferably, may be the mean square error.
An adaptive equalizer structure for high-rate transmission systems is described by H. Kobayashi in "Simultaneous Adaptive Estimation and Decision Algorithm for Carrier Modulated Data Transmission Systems", IEEE Transactions on Communication Technology, Vol. COM'19, No. 3, pages 268-280, June 1971.
The author describes a receiver structure in which the timing and carrier recovery, and also the adaptation of the digital equalizer, are effected with the aid of a minimum mean square error criterion. In this structure, the signal is first demodulated, thereafter sampled and then processed by the adaptive equalizer. However, to effect the timing recovry ensuring an optimum sampling phase, in the receiver structure described by H. Kobayashi, the input signal x(t) is first differentiated and thereafter processed in parallel with the main path in accordance with the same principles, using a second sampler and a second adaptive equalizer having characteristics identical to those of the main adaptive equalizer. A correlation circuit is thereafter provided at the output of the timing recovery loop to minimize the mean square error function chosen and to intervene in the phase of the sampling instant.
In a concrete situation it was found to be difficult to utilize this structure as actually it means doubling the complexity of the receiver.